In completely photonic systems, the only known way to store information carried by photonic signals, e.g. modulated light, is to direct the signal along an optical delay path which returns the signal to the desired output location after the delay required for the photonic signals to traverse the delay path. As the photonic signals travel along a passive optical delay path they typically become attenuated. Thus, if a group of photonic signals that have traversed a delay path are inserted into a data frame just before or just after a group of photonic signals that have not been delayed, there typically is a substantial difference between the intensities of the two photonic signal groups. FIG. 1 shows an optical slot interchange system that provides such delays and the problems of different photonic intensities within a resulting resulting data frame. Differences in intensity between photonic signal groups cause difficulties in demodulation and detection of the data carded by the signals because most optical demodulators operate best with inputs that have uniform intensity levels.
U.S. Pat. No. 5,214,729 issued May 25, 1993 to Kwang-Tsai Koai mentions the problem of differences of intensities and the detrimental effect which the problem has on system performance. As a solution, Koai suggests changing the splitting ratio of a passive input splitter and/or the combining ratio of the combiner within the optical time slot interchanger. Since the Koai patent considers passive splitters passive and combiners, these splitting ratios would be fixed to equalize all of the photonic intensity levels with the intensity of the longest delay path, even if that path is not used for the processing of a data frame. Technically, the combiners may be considered active instead of passive since they have amplifiers in series with them that have two states, block and amplify by a fixed dB amount. This off-or-amplify-by-a-fixed-amount characteristic does not change the fact that each path from combiner to output has a fixed attenuation. With this fixed system of Koai, some photonic signals will be attenuated more than the amount necessary to equalize the differences in their respective frame of optical data because the attenuations are selected to equalize the level the photonic intensities to that of the worst case delay. Because most frames will not need the worst case intensity equalizing, the signal to noise ratio of the system of Koai is virtually always lower than it could be if splitters and combiners with fixed attenuation were not used.
It is an object of the present invention to provide an apparatus that equalizes the photonic intensities of the optical data slots in each frame, such that all photonic intensities are equal to the intensity level of the most attenuated slot of the frame, whatever that level might be, thereby achieving the maximum signal to noise ratio possible for an equalized frame and improving the detectability of the frame by an optical detector.
It is an object of the present invention to provide an apparatus that attenuates each optical data slot by the minimum amount required to equalize the differences of photonic signal intensities within each frame of optical data.
It is another object of the present invention to provide a method for attenuating the intensity of each optical data slot by the minimum amount required for equalizing the differences of photonic intensities within each frame of optical data.